The bleak and unforgiving borderlands of Texas by the Rio Grande are the setting for this triumphant new movie by Joel and Ethan Coen, based on the western thriller by Cormac McCarthy. It's their best since The Man Who Wasn't There in 2001 - and it's the best of their career so far. The Coens are back with a vengeance, showing their various imitators and detractors what great American film-making looks like, and they have supplied a corrective adjustment to the excesses of goofy-quirky comedy that damaged their recent work. The result is a dark, violent and deeply disquieting drama, leavened with brilliant noirish wisecracks, and boasting three leading male performances with all the spectacular virility of Texan steers. And all of it hard and sharp as a diamond.

The setting is 1980, though the period is not signalled with any of the traditional giveaways. Tommy Lee Jones plays Sheriff Ed Tom Bell, an intelligent and sympathetic lawman, from a proud family of lawmen, who is, however, preparing to quit, having become disenchanted by society's inability to contain the criminals' evolutionary leap to a new level of ruthlessness. The twang and roll of Jones's voice is controlled with a musician's flair and the craggy folds of his hangdog face are a Texan landscape in themselves. He has a goosebump-inducing opening voiceover about sending unrepentant young killers to the gas chamber, superimposed on prospects of the western terrain photographed by Roger Deakins; it recalls the famous aria at the top of the Coens' first film, Blood Simple, in 1984.

Sheriff Ed Tom is nonetheless a welcome voice of sanity and humour in a world of evil. When he stumbles across a ring of decaying corpses and shot-up trucks and SUVs in the remote desert - the grisly remains of a drug deal gone sour - his callow young deputy remarks, plaintively: "It's a mess, ain't it, Sheriff?" And Ed Tom replies, tersely: "If it ain't, it'll do until the mess gets here."

The second player is the psychopathic hitman Anton Chigurh, played by Javier Bardem, who has been hired by shadowy interests to recover a satchel of $2m in cash that has gone missing after this failed drug deal. Chigurh has an appalling hairstyle, and a habit of killing people with the air-pressure boltgun generally used on livestock, but there is nothing funny or ironic about him; he has a fanaticism that goes beyond the icy commitment of an assassin for hire. He is effectively a serial killer, an existential devotee of murder, a connoisseur of fear and victimhood, and finally appears to forget about the money in his pursuit of slaughter. The scene in which he bullies and threatens the gentle old proprietor of a local gas station is scalp-pricklingly disturbing. (Has the Coens' pre-eminence at the Cannes film festival caused them to imbibe the style of fellow auteur Michael Haneke?)

The third player is the Coens' great find: they have made a star of him. Josh Brolin is absolutely superb in the plum role of Llewelyn Moss, the taciturn Vietnam veteran who has innocently stumbled across this drug money while out hunting and headed off over the Mexican border with it, followed by the sinister Chigurh and the long-suffering Ed Tom, who knows that Moss is just a good ol' boy who needs protection and whose decent wife, Carla Jean, (Kelly Macdonald) is worried sick about him. Brolin gets some of the best lines in the film. "If I don't come back, tell my momma I love her," says Llewellyn. "But your momma's dead," replies Carla Jean. "Well, then I'll tell her myself," says Llewellyn, after a thoughtful pause.

The tone of the film, like that of McCarthy's original novel, is apocalyptic: it gestures ahead, darkly, to an utter annihilation of norms and restraints. The Coens' adaptation in fact omits the details of Ed Tom's experiences in second world war and with it some of the Sheriff's internal life and his need for redemption, but this omission has the effect of intensifying the motiveless, ahistorical quality of the action, the sense that the contest between the good guys and the bad guys under the Texan sun has become even more eternally brutal. The Coens are true to the pessimistic severity of the book's ending - darker, arguably, than the ending of McCarthy's great novel The Road, to whose horror this story can, in retrospect, be seen to be heading.

The savoury, serio-comic tang of the Coens' film-making style is recognisably present, as is their predilection for the weirdness of hotels and motels. But in McCarthy's novel they have found something that has heightened and deepened their identity as film-makers: a real sense of seriousness, a sense that their offbeat Americana and gruesome and surreal comic contortions can really be more than the sum of their parts.

Tommy Lee Jones and the actor Barry Corbin have a wonderfully modulated scene, in which Ed Tom calls on his old Uncle Ellis, another retired police officer, who has seen enough of the unequal struggle against evil to have even fewer illusions than his nephew. But he tells him that America has always been like this, that it is a tough country, cruel and harsh, eating its sons like Saturn. Watching this film has something of the elemental thrill of watching a cloud-shadow spread with miraculous speed over a vast, empty landscape: it has a chilly, portentous intuition of what America is.